Researchers have discovered how to transform human embryonic stem cells into germ cells, a type of embryonic cell that gives rise to sperm and eggs.

The breakthrough could ultimately lead to research that would help infertile couples.

The work did not create sperm or eggs, but the advance will allow researchers to observe human germ cells in laboratory dishes, something they otherwise haven't been able to do.

"Ten to 15 percent of couples are infertile," said senior author Renee Reijo Pera of Stanford University. "About half of these cases are due to an inability to make eggs or sperm. And yet deleting or increasing the expression of genes in the womb to understand why is both impossible and unethical. Figuring out the genetic 'recipe' needed to develop human germ cells in the laboratory will give us the tools we need to trace what's going wrong for these people."

The results were published online in Nature on Oct. 28.

"This achievement opens a new window into what was only recently a hidden stage of human development," said Susan B. Shurin, acting director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, a federal institute that provided funding for the study. "Laboratory observation of human germ cells has the potential to yield important clues to the origins of unexplained infertility and to the genesis of many birth defects and chromosomal disorders."

Researchers have long sought to understand the process by which cells in the early human embryo mature into germ cells, Reijo Pera explained in a statement. But studying this process in human beings has been impossible in the past, because it takes place so early in development—before the embryo is two weeks old.

Although infertility is apparent only after sexual maturity, she said, many forms of unexplained infertility are thought to have their origins in errors that occur in the cells of the early embryo. The ability to observe embryonic germ cells as they develop may allow researchers to pinpoint potential genetic changes underlying infertility.